In the manufacture of free-standing solid materials by chemical vapor deposition (CVD), reactants in the form of gases are introduced into a hot furnace or deposition chamber where they react on the walls of the furnace or on the surface of a substrate or mandrel positioned in the furnace to form a solid deposit or coating thereon which when removed from the substrate or mandrel is free-standing. Typically a vacuum furnace designed in the shape of a tubular cylinder having a circular cross section is used for chemical vapor deposition. Mixing chambers are also provided which are designed to ensure that the gases and other reagents are mixed together to provide a uniform mixture. In many conventional mixing chambers the gases are injected through two or more injectors flowing in parallel or cross configuration from one wall of the mixing chamber. When the length of the mixing chamber is of the same order as the width, the gases do not have sufficient residence time in the mixing chamber before they exit into the deposition chamber. This results in a non-uniform mixing of gases.
The mixed gases are injected into the deposition chamber at relatively fast rates in order to avoid undesired powder formation due to the fast reactions between the gases. However, this causes non-uniform mixing within low aspect ratio deposition chambers and results in low deposition rates on the walls of the chambers. In the formation of spinel where chlorides and water are typical reagents, the reaction between the chlorides and water is very rapid and powder still forms regardless of the fast injection rates of the gases into the deposition chamber. Accordingly, there is a need for an apparatus and method for reducing powder formation and improving chemical vapor deposition of solid materials.